In an acoustic apparatus having a microphone and a loudspeaker, a howling component may be generated as a result of the fact that a sound is outputted to the microphone from the loudspeaker as a feedback.
Up until now, there have been proposed a wide variety of howling suppressing apparatuses of this type, one typical example of which is adapted to extract a howling component from an input signal by using an adaptive filter, and to suppress the howling component by subtracting the howling component from the input signal (see, for example, patent document 1).
The following description is directed to the conventional howling control apparatus shown in FIG. 10.
As shown in FIG. 10, a delay unit 2 is adapted to delay an input signal received from a microphone through an input terminal 1. An adaptive filter 3 is adapted to extract a howling component from the signal delayed by the delay unit 2. An adder 4 is adapted to suppress the howling component by subtracting the howling component extracted by the adaptive filter 3 from the input signal, and to output the suppressed signal through an output terminal 5.
The operation of the conventional howling control apparatus disclosed in the patent document 1 will be described hereinafter with reference to FIG. 10.
In the howling control apparatus disclosed in the patent document 1, an input signal from the input terminal 1 is delayed by the delay unit 2. The convolution of the signal delayed by the delay unit 2 and filter coefficients is then performed by the adaptive filter 3. A signal delayed by a delay time equal to an integral multiple of the cycle of the howling component of the input signal is then generated by the adaptive filter 3.
The adder 4 suppresses the howling component of the input signal by subtracting the output signal of the adaptive filter 3 from the input signal, and outputs the suppressed signal to the adaptive filter 3 and an output terminal 5. The adaptive filter 3 updates the filter coefficients successively to minimize a mean square of the output signal of the adder 4.
The conventional howling control apparatus disclosed in the patent document 1 can suppress the howling component of the input signal by extracting the howling component from the input signal by using the adaptive filter 3, and subtracting the howling component from the input signal.    Patent document 1: Japanese published unexamined application No. 2001-275182 (see Page 5, FIG. 1)